296 MR PHILIP J. WHITE ON THE 



A small shelf of cartilage (Io), with a transverse groove on its upper surface, lies 

 over the carotid opening, and at either end of the groove the inner opening of the 

 inter-orbital canal is placed. In the position of the shelf, the continuity of the 

 inter-orbital canal as a cartilaginous tube is broken above, a condition which is found 

 in Hexanchus and some other sharks. The portion of the canal on each side of the 

 pituitary fossa, in Lsemargus, pursues from without, a curved course inwards and 

 slightly forwards. 



The Hypophysis canal. — The internal opening of this canal (He), already alluded 

 to, and which is presumably the persistent hypophysis canal, is seen on the anterior 

 wall of the pituitary fossa. The canal has an oblique course passing upwards and back- 

 wards from below. In one skull which I examined the canal had a vertical direction, and 

 passed directly upwards to open at the lowest part of the fossa. Hexanchus and Hep- 

 tanchus seem to have a canal which, while not so complete as in Lsemargus, apparently 

 represents it in these forms. 



Two openings, a dorsal and a ventral, for the branches of the hyoid artery, are placed 

 some distance apart on the postero-lateral walls of the fossa. 



The foramen (Qm)for the oculi-motor nerve, as already indicated, lies at the forepart 

 of an elevation which passes outwards and upwards from the dorsum sellse. 



The optic foramen (0) is situated in a line with, but considerably in front of, that 

 for the oculi-motor nerve. 



The patheticus foramen (Pa). — This is placed on the dorso-lateral wall of the cranial 

 cavity, some distance above the optic foramen, and slightly posterior to it. 



The anterior division. — This division of the cranial cavity lies in front of the optic 

 foramina. It is incompletely shut off from the pre-frontal fossa (P/*) by the cartilaginous 

 partition (D), and on either side of this partition the cavity is produced as a wide 

 olfactory passage. Each olfactory passage, although it is wide throughout, becomes 

 narrower as it passes forwards and outwards to open at the hinder part of the nasal 

 cavity. A foramen (z) for a blood-vessel is placed at the dorso-lateral part of this 

 cranial division, near the commencement of the olfactory passage. 



The partition (D), which incompletely separates the cranial cavity from the pre- 

 frontal fossa (P/), rises vertically from the cranial floor. In the skull figured, the partition 

 is only connected at its lower part with the cranium by means of a narrow neck of 

 cartilage, and as it does not touch the cranium at any other part, a space exists between 

 it and the cranial wall, by means of which the cranial cavity and pre-frontal fossa 

 communicate freely with each other. In another skull, the partition had three connec- 

 tions with the cranium. There was a broad connection at its base with the cranial floor, 

 and its upper part was connected with the cranial roof by two narrow bands of 

 cartilage, one on either side. In this case, therefore, instead of a single space existing 

 between the cranial cavity and pre-frontal fossa, there were three, — one dorsal, and one 

 on either side. In a third skull, the largest cranium which I examined, not only 

 was there an extensive basal continuation of the partition with the cranial floor, but 



